Abstract

Three–dimensional dynamic stall was observed on the rotor of a fully equipped sub–scale helicopter model in a wind tunnel. Results of stereoscopic particle image velocimetry measurements on the retreating blade clearly revealed the large–scale dynamic stall vortex at 50% and 60% blade radius. The small-scale shear layer vortices that constitute the large–scale dynamic stall vortex were identified and localised. Based on the spatial distribution of these small–scale vortices, the characteristic large–scale structure was found to be unexpectedly compact. Along with an apparent spanwise flexion of the dynamic stall vortex, these results suggested the rotational motion of the rotor to have a stabilising effect on the formation and convection of the dynamic stall vortex. The velocity field information available provides a basis for validating recent computational fluid dynamics results of three–dimensional dynamic stall.